Lamp having plurality of optical extraction structures

ABSTRACT

A lamp for emitting light, comprising a transparent sheet-like lightguide, with at least one light receiving side, a light emission front surface and a back surface opposite the front surface. The lamp further comprises a plurality of light sources, positioned in an array and optically coupled to at least one light receiving side. The back surface of said lamp comprises a plurality of optical extraction structures, for example provided in parallel curved lines. Furthermore, the lamp is substantially free from light scattering structures in a light path of the light to be emitted.

This application is a national stage application under 35 U.S.C. §371 ofInternational Application No. PCT/IB2007/054046 filed on Oct. 5, 2007,which claims priority to European Application No. 06121985.3, filed onOct. 9, 2006, incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates in general to a lamp. In particular, the inventionrelates in a first aspect to a lamp for emitting light, comprising atransparent sheet-like lightguide, with at least one light receivingside, a light emission front surface and a back surface opposite thefront surface, and a plurality of light sources, positioned in a lineararray and adjacent at least one light receiving side, wherein the backsurface comprises a plurality of reflective optical extractionstructures.

BACKGROUND OF THE INVENTION

So-called backlights of e.g. liquid crystal displays generally have asimilar construction, comprising a side-lit lightguide, and extractionfeatures to extract the light out of the lightguide. Such backlights maye.g. be relatively flat lamps that still have a large light emittingarea. Often, these backlights have to fulfill the requirement ofproviding lighting as homogeneously as possible, and in particular thelight extraction structures are adapted to this goal.

However, a disadvantage of such backlights, or lamps of this kind ingeneral, is that the direction of the emitted light is much lesswell-defined. A desirable control over lighting properties cannot beobtained with such lamps with homogeneously, yet substantially randomly,emitted light.

It is an object of the present invention to provide a flat lamp havingthe known advantages of the known lamp, but that in addition is able toemit light with a better-defined behavior. This better-defined behaviorin turn can provide special properties hitherto not supplied by e.g.backlights.

SUMMARY OF THE INVENTION

The above object is achieved with a lamp according to claim 1, inparticular a lamp for emitting light, comprising a transparentsheet-like lightguide, with at least one light receiving side, a lightemission front surface and a back surface opposite the front surface,and a plurality of light sources, positioned in an array and opticallycoupled to at least one light receiving side, wherein the back surfacecomprises a plurality of optical extraction structures, and the lamp issubstantially free from light scattering structures in a light path ofthe light to be emitted. By leaving out light scattering structures,such as diffuser foils or diffusive (scattering) particles, the lampaccording to the invention provides a flat lamp with a large lightemitting area, wherein the light has well-defined properties. Theseproperties relate particularly but not exclusively to a well-definedpolarization state, which allows various manipulations, as will be shownbelow.

It is noted that in this respect a liquid crystal device is alsoconsidered to be a light scattering structure. In general, a displaycomprising a backlight and a liquid crystal device is not intended to becomprised in the scope of the present invention.

In a special embodiment of the lamp according to the invention, thefront surface, the extraction structures and the back surface compriseoptically flat surfaces. Such optically flat surfaces help inmaintaining a well-defined polarization state, since e.g. a varyingthickness of the lightguide or an irregular surface could alter thatpolarization. In this respect, a surface is called optically flat if ithas an average surface roughness R_(a) of less than 2% of the wavelengthof the used light. Since for optical applications with visible light theaverage wavelength is about 500 nm, the surfaces are optically flat ifthat roughness R_(a) is at the most about 10 nm, preferably smaller suchas smaller than 5 nm. Obviously, the back surface is taken to be thatpart of the back surface that extends around the extraction structures,which are inherently needed to extract light from the lightguide.

In a particular embodiment, the extraction structures comprise mirroringsurfaces. By suitably positioning the mirroring surfaces, light that isguided in the lightguide by total internal reflection may be reflectedsuch that it is able to leave the lightguide. Depending on the index ofrefraction of the material of the lightguide, and on the desired exitangle, the angle of the mirroring surfaces with respect to the backsurface may be selected. Suitable angles are between about 30 and 60°,preferably around 45°. The mirroring surfaces may comprise surfacesmachined or recessed into the lightguide, or may comprise externalsurfaces, such as prism surfaces.

In a special embodiment, the extraction structures are mutuallyseparated by a distance of at least 2 mm, preferably at least 5 mm. Thisrelates to a special effect that was discovered by the inventors. Whenlooking at the lamp, the extraction structures, i.e. for example themirroring surfaces, will reflect the light from the light sources. Thiswill cause a virtual image of each of the light sources to be made forevery extraction structure. These images will have an intensity thatdepends on the dimensions of the extraction structure, e.g. an intensitythat diminishes away from the real light sources, if the extractionsurfaces are all equal. A more important effect as found by theinventors is that, especially when looking at the lamp at a non-zeroangle with respect to a plane perpendicular to the array of lightsources (e.g. if the array of light sources are at the lower side of thelamp, when looking from the sides), the virtual images of the lightsources seem to “run off” into the distance. This means that, thefurther the viewer is from the real light sources, the deeper into thelamp the virtual images are perceived. This means that the flat lamp hasa perceived depth. This 3D-effect may be observed because the light asextracted by the extraction structures is well-defined. The individualimages of the light sources would be blurred into one mass of light ifthe lamp were to comprise light scattering means, or irregularextraction surfaces et cetera. Now, with the above mutual distancebetween the extraction structures, it is easily possible for the viewerto separate the separate virtual images. If the virtual images were tooclose together, they would coalesce into a single light field, whichwould suppress the 3D-effect.

A suitable height of the extraction structures would be up to about 25μm, preferably between about 5 and 15 μm. This allows sufficient lightto be extracted, without interfering too much with the guiding of lightwithin the lightguide. Note however that this relates to lightguides ofsay 2 mm thickness. The dimensions may scale up or down with thelightguide thickness, and a more general rule could be that the heightof the structures could preferably be about 0.1-2% of the thickness ofthe lightguide.

Furthermore, it is to be stressed that this 3D-effect is alsoperceivable if there is only one light source, either one LED or othersmall light source, and also in the case of a single linear lightsource, such as a thin fluorescence lamp. However, this is much lessclearly visible, inter alia because variations in intensity along thearray of light sources are not present (or very much less so), and italso depends more strongly on side effects, as will be discussed furtherbelow.

In a particular embodiment, the extraction structures are provided oncurved lines. In this embodiment, the virtual images of the lightsources also seem to lie on curved lines. In this way, there can beprovided a lamp with various simulated depth profiles. These lines maybe parallel lines, including concentric lines. Also other groups oflines or line structures are possible, depending on the desired (3D)appearance of the lamp.

In a special embodiment, the sides of the lightguide that extendperpendicularly to the array of light sources are substantiallynon-reflecting. This helps in obtaining the perceived depth effect.Since, in this case, the sides do not reflect light, no virtual images“to the sides” of the lamp are generated. This means that, when lookingat the lamp from the side, at the far end of the lamp a dark part willbecome visible, at which part no virtual image is present to emit light.In the case of a rectangular front surface of the lightguide and ofextraction structures lying on straight lines, the dark part will be atriangle. With other shapes, or curved lines for the extractionstructures, the dark part may have a different shape. It is especiallythis dark virtual side wall which enhances the perceived depth, all themore so since this dark part only appears when looking from the sides.When shifting the viewpoint from “right in front” to “from the side”, atfirst there is no dark part, but it gradually grows as the viewpointshifts, just as would a real dark sidewall. Note that the effect becomesalso visible when the respective sides of the lightguides reflectvisibly less than 100% of the light, such as 80% or less. However, forthe effect to be impressive, it is advantageous if the reflectance is atmost 25%, and preferably as small as possible, such as 5% or less.

In an advantageous embodiment, the plurality of light sources comprisesa plurality of LEDs, in particular a plurality of white light LEDs. LEDsprovide a bright and useful all round light that can easily be coupledinto the light guide, due to their small dimensions.

In a particular embodiment, the plurality of light sources comprises aplurality of LEDs, at least two neighboring LEDs being arranged to emitlight of a different color. This allows an even stronger 3D perception,since the virtual images will now become quite clearly visible, e.g. ascolored lines perpendicular to the curves on which the extractionstructures lie; in many cases these lines extend perpendicularly to thearray of light sources.

The light sources need not be LEDs, but could also be other spatiallyseparated light sources. In a particular embodiment, the light sourcecomprises a central light generating means and a plurality of opticalfibers. The light generating means could again be an LED, but also anyother “lamp”, such as an incandescent lamp, a (high pressure) dischargelamp, etc. Because of the optical fibers, this light generating meansmay be positioned away from the lamp of the present invention, i.e. fromthe lightguide, which may still be flat. The other ends of the opticalfibers may then be considered the light sources in the lamp according tothe present invention.

In a special embodiment, the plurality of light sources comprise alinear light source, preferably a fluorescent lamp, further comprising alight division structure adjacent the fluorescent lamp, arranged todivide a light emitting surface of the fluorescent lamp into a pluralityof light emitting sub-surfaces. Such a light division structure could bea mask, or the like, such as an opaque foil with one or more apertures.In this way, a single light source may be used, which decreases thecomplexity of the lamp. Furthermore, different effects are easilyobtainable by simply changing the light division structure, withouthaving to change anything about the light source (or an array of lightsources).

In a particular embodiment, the array of light sources comprise anarrangement which extends in more than one dimension, in particular morethan two dimensions. In a special embodiment, a plurality of lightsources are present on a curve. Especially, the arrangement comprises atleast two rows of light sources. This allows the creation of additional3D effects, in that suitable positions for the light sources may beselected, such that their virtual images may be observed in desiredpositions.

In an even more special embodiment, at least one light source ismoveable in at least one direction, with respect to the light guide.Note that this may also be an aperture of the light division structure.By providing the light sources on a curve, the virtual images may alsobe provided on a curve, which is itself mirrored in the extractionstructures. By making at least one light source moveable, the virtualimages may also be made moveable. All this allows the creation of more3D effects, still without increasing the thickness of the lamp. The lampmay advantageously comprise at least one light source actuator, formoving the light source(s). Such actuators may comprise piezo-electricalactuators, or other mechanical, hydraulic, magnetic actuators. Theactuators may move the light sources themselves, such as the LEDs or the(ends of the) optical fibers, or may move, rotate etc. the lightdivision structure.

Alternatively, the light sources could also comprise a light emittingdevice, of any kind mentioned above, and one or more, preferablymoveable, mirrors. In this way, too, the virtual images may be made tomove in the eyes of the viewer.

In an advantageous embodiment, the lamp comprises a circular polarizerpositioned in front of the front surface and a mirror positionedadjacent the back surface. This further enhances the 3D effect, in thatit effectively suppresses reflections of ambient light. As is known tothe skilled person, incident ambient light is blocked by the circularpolarizer for the part with a selected polarization. The remaining partwith the complementary polarization will pass the polarizer. At themirror, the light is reflected while at the same time the polarizationis flipped. This light will subsequently also be blocked by thepolarizer. In all, substantially all of the incident light is absorbedby the polarizer. This means that the effects visible in the lamp arevery pronounced, and not blurred by reflections of ambient light.

It is to be noted that with the above feature, there is also provided aflat lamp in general, that has a very low reflection, and thus a veryhigh contrast between brightness in the “on” and the “off” state. Onecould also state that the visibility of the lamp is improved. To achievethis, the invention provides a lamp for emitting light, comprising atransparent sheet-like lightguide, with at least one light receivingside, a light emission front surface and a back surface opposite thefront surface, and at least one light source, positioned adjacent alight receiving side, the back surface comprising a plurality of opticalextraction structures, and the lamp being substantially free from lightscattering structures in a light path of the light to be emitted, andthe lamp comprising a circular polarizer positioned in front of thefront surface and a mirror positioned adjacent the back surface.

In the above embodiments, it is also advantageous when the lamp issubstantially free from light scattering structures in a light path ofthe light to be emitted, since that ensures that the polarization of theambient light that passes the circular polarizer is not changed, apartfrom the flip at the mirror. This in turn ensures full absorption of theambient light.

In a particular embodiment, the lightguide has a retardation of lessthan 20 nm. This ensures an extinction rate of at least 98% of theambient light. Of course, a retardation that is as low as possible ispreferred.

In an advantageous embodiment, the front surface is directly visible.This is to be taken to mean that the front surface is substantially freefrom, i.e. not covered by, a structure that prevents the front surfacefrom being seen by an observer. It is not intended to comprisetransparent structures that still allow a view of that front surface.All this relates in particular to separate structures of the kindmentioned above, namely diffusers, liquid crystal devices and so on.

In a particular embodiment, the lightguide comprises parallel first andsecond lightguide parts. This further broadens the gamut of effectsobtainable. E.g., the lightguide parts may be provided side-by-side,with different extraction structures, or may be lying on differentcurves/lines. It is also possible to provide the lightguide parts so asto be overlapping, i.e. having at least partly overlapping respectivefront surfaces. In such a case, virtual images as created by thelightguide parts may also be overlapping. E.g., in an advantageousembodiment, the lamp further comprises two arrays of light sources, andrespective light receiving sides of the first and second lightguideparts are optically connected to one of the two arrays of light sources.

In a particular embodiment, the two arrays of light sources are arrangedon different, preferably opposite, sides of the lamp. This may simply beused to enhance the brightness of the lamp, but also to add variouseffects in one lamp.

In another embodiment, the first lightguide part has a light receivingside positioned adjacent the array of light sources, and the secondlightguide part is optically connected to the first lightguide part bymeans of a mirror device. In this embodiment, one array of lightsources, or one light source, suffices to provide two lightguide partswith light. The optical connection may be embodied in any desiredfashion, such as fiber optics. Preferably however, the connection isestablished by means of a suitable prism, through total internalreflection.

In an advantageous embodiment, the first lightguide part issubstantially free from extraction structures. In this embodiment, thelight from the light source(s) first passes the first lightguide partwithout being extracted. On extraction in the second lightguide part,the perceived depth of the virtual images is, however, larger than wouldbe the case for extraction in the first lightguide part. This ensuresthat the perceived depth increases. Of course, it is possible to keepincreasing the depth by adding more and more lightguide parts with asuitable optical connection, although there will be some intensity loss.Furthermore, it is also possible to provide also the first lightguidepart with extraction structures, which would then also create virtualimages.

In a particular embodiment, the first and second lightguide partscomprise a part at respective, mutually opposite light receiving sidesthereof that is free from extraction structures. In particular,substantially half of the respective lightguide part is free fromextraction structures, although other ratios are also possible. In anadvantageous embodiment, the lamp according to the invention furthercomprises two arrays of light sources, and two lightguides withextraction structures and with opposite light receiving sides. In such acase, it is possible to provide a lamp having a perceived depth that hasa minimum in the center of the lamp, in particular with overlappinglightguides and opposite arrays of light sources.

The invention also provides a lamp for emitting light, comprising atransparent sheet-like lightguide, with at least one light receivingside, a light emission front surface and a back surface opposite thefront surface, and at least one light source, positioned adjacent alight receiving side, and a circular polarizer positioned in front ofthe front surface, a mirror being provided which is positioned adjacent,and preferably in contact with, the back surface, the front sidecomprising a plurality of optical extraction structures, the extractionstructures comprising recesses in the lightguide that are filled with abirefringent material, and the lamp further comprising additionalbirefringent material that is positioned between the circular polarizerand the birefringent material of the extraction structures. Thisprovides the possibility to influence on the one hand the ambient light,along the same optical mechanism as described above, and on the otherhand to influence the amount of light that is extracted by thelightguide. In principle, without the additional birefringent material,the following would happen.

The birefringent material in the recesses should be selected such thatits refractive index for either the fast or the slow axis issubstantially equal to that of the refractive material of thelightguide. In that case, the part of the light with the suitablepolarization ‘sees’ only a lightguide and passes the filled recessunhindered, while the part with the opposite polarization can berefracted out of the lightguide. The unhindered part then reflects atthe mirror and can be made to flip its polarization, which means that atthe next encounter with a recess, it will leave the lightguide. Hence,all light can leave the lightguide, and it will all have the same linearpolarization. Half of this extracted light would then be absorbed by thecircular polarizer. If the additional birefringent material is nowprovided with a suitable thickness, i.e. retardation, and orientation ofthe fast (or slow) axis, the linearly polarized light can be made topass the circular polarizer unhindered, while ambient light is stillfully absorbed.

This provides a lamp having about 100% light extraction efficiency,while still suppressing reflection of ambient light.

Note that exactly the same structure as described above may be providedin the other lamps according to the invention.

In a special embodiment, the lamp is substantially free from lightscattering structures in a light path of the light to be emitted, inparticular, but not exclusively, it is free from LC devices. Again, theembodiment has optimum efficiency if changes to the polarization stateare at a minimum.

The invention also provides the use of a lamp according to the presentinvention for illumination in a building. The use of the inventive lampsprovides great advantages, e.g. in that lighting may be provided whichis not reflecting by itself. In other words, if the lamp is off, itappears rather black. This offers advantages in circumstances wherethere should be little light, while the possibility of providing lots oflight if desired is available. This is for example the case in (movie)theatres, clubs, discotheques et cetera. Known lamps would comprise somekind of reflector, diffuser etcetera, which shine whitish even when thelight source is off. This is not desired in the establishmentsmentioned.

Other possible uses include the use in a building where e.g. only asmall space is available. Because of the 3D effect, the impression of alarger room may be given by the flat lamp according to the invention.

In particular, the lamp is mounted on or in a wall, floor or ceiling ofthe building. Alternatively, the lamp is mounted in a base and ispositioned in free space. In both cases, the space occupied by the lampis small, while the impression of spaciousness can be rather high, whichmay be desired in cases where the total actually available room issmall, like inside an elevator

A particularly advantageous use according to the invention is, inparticular, in an outdoor environment. In such a case, a flat lamp isprovided that has a very small reflection of ambient light (glare),which ensures that the light emitted by the lamp itself is bettervisible. In particular, the lamp is used in a traffic sign, such as atraffic lamp. Alternatively, the lamp may be used in other signs forwhich improved visibility is desired, such as advertisements, warningsigns et cetera.

The invention also provides a building provided with a lamp according tothe invention, with the advantages already mentioned above. Inparticular, the lamp is provided on a wall, and in particular not behinda screen, i.e. without LCDs. In other words, LCD displays such as TVsets are excluded here.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 very diagrammatically shows a lamp according to the invention, ina cross-sectional side view, with a 3D appearance.

FIG. 2 shows a diagrammatic front view of a lamp according to theinvention.

FIG. 3 shows a perspective view of a lamp according to the invention.

FIG. 4 diagrammatically shows another embodiment of the lamp of theinvention.

FIG. 5 shows another embodiment of the lamp of the invention, comprisingfirst and second (arrays of) LEDs 1-1 and 1-2, lightguides 2-1 and 2-2,extraction structures 5-1 and 5-2, creating virtual LED images 10-1 and10-2.

FIG. 6 diagrammatically shows a cross-sectional view of anotherembodiment of a lamp according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 very diagrammatically shows a lamp according to the invention, ina cross-sectional side view, with a 3D appearance.

Herein, 1 denotes an LED and 2 a lightguide with a front surface 3 and aback surface 4. In the lightguide there are a plurality of extractionstructures 5 and 6. A mirror is denoted with reference numeral 7.

LED 1 emits a (narrow) bundle of light 8 that enters the lightguide 2.The light is guided by total internal reflection at the front surface 3and the back surface 4. If necessary, the light is reflected back bymirror 7.

A part of the light will be incident on the extraction structures 5,either directly or after a number of reflections, as partly indicated bydashed lines. The extraction structures 5 are recesses that are e.g.machined into the lightguide, and that comprise a reflection surface atan angle e.g. between 30 and 50°. The light that is incident on theextraction surfaces is reflected to the front surface 3 and is able toleave the lightguide 2. Of the bundles that thus leave the lightguide 2,a number of parallel rays 9 are shown, although in fact each extractionstructure emits also a bundle of rays. However, these rays 9 indicateone of the directions from which light is perceived by a (distant)viewer.

To a distant viewer, the rays 9 will be perceived to be emitted byvirtual images 10 of the LED 1. Each of the extraction structures 5 willprovide one virtual image. The further away the extraction structure 5is from the original and real LED 1, the more distant the virtual image10 will appear. This way, the lamp will be perceived as having a depth,although the lamp is of course actually flat.

The extraction structures 6 have a symmetrical structure, which allowsthem to also reflect light coming from above, i.e. light reflected bythe mirror 7. Although this allows more light to be extracted, it alsocreates a second virtual image at roughly the same vertical position asa virtual image that is created by light coming from below, but at adifferent perceived depth. This may blur the actually perceived depth,so the mirror is only optional.

In this context, it is important to note that the lightguide 2 isessentially free from scattering structures, such as incorporatedparticles or a diffuser screen or the like. Likewise, the surfaces ofthe extraction structures 5 should also be non-scattering, and in thiscase they should be optically flat. In this way, directions et cetera ofthe light are always well-defined.

FIG. 2 shows a diagrammatic front view of a lamp according to theinvention. An array of LEDs is denoted 11 ₁, 12 ₁, 13 ₁, 11 ₂, 12 ₂, 13₂, 13 ₁, 13 ₂, 13 ₃, . . . ; virtual images are shown on substantiallyvertical lines or curves 14. Curves of extraction structures are denotedby 15.

In this case, the extraction structures do not lie on straight lines buton concentrical lines. This causes the virtual images 14 not to lie onstraight vertical lines, but also on curved lines, as shown here. Thesecurves may help in enhancing the depth perception. The curves 15 mayalso be composed of a number of shorter lines or curves, and could havemany different shapes.

The light sources are an array of LEDs, in this case 3 LEDs for eachlight source, such as a red, a green and a blue LED, that together mayemit white light. Note however that the virtual images in the lamp arenot blurred, and that at not too far distances the lamp will appearcolored. Alternatively to the RGB-LED groups, e.g. white light LEDs maybe used, or a thin lamp with a suitable ‘mask’, or optical fibers etcetera.

The light sources are shown as being arranged on a single straight line,but they could be provided on more, parallel lines, or other groups andarrangements. Furthermore, they could also be made so as to be moveable,for example in the x and y direction as indicated by arrows. To thisend, suitable actuators could be provided (not shown here). The virtualimages in lines 14 could then also move, i.e. in the eyes of the viewer.

FIG. 3 shows a perspective view of a lamp according to the invention.The lamp comprises a housing 16 with two apertures for two sub-lamps.Each sub-lamp comprises an array of LEDs (not directly visible), each ofwhich projects a series of virtual images, visible as more or lessvertical lines 14. Likewise, the array of LEDs is imaged as a more orless horizontal series of virtual images 20, between which narrow darkerbands are visible.

Furthermore, a dark “virtual sidewall” 21 is visible. The virtualdimensions of this dark sidewall 21 vary as the angle at which a viewerlooks at the lamp is changed, just like a lamp with a real depth. Thiscontributes very much to the “depth” of the lamp according to theinvention. The visibility of this sidewall 21 may be improved by makingcorresponding sides of the lightguide non-reflecting, e.g. absorbing. Ifthe sides were reflective, further images towards the sides of the lampwould be created, which would take away the dark sidewall.

The dark vertical bands between the virtual images may be made largerand smaller. To do so, the individual LEDs need to be positioned furtherapart. The horizontal bands may be made larger by positioning theextraction features further apart, or by limiting the angles at whichtotal internal reflection can occur, because in that case the bundle oflight in the lightguide will be narrower.

FIG. 4 diagrammatically shows another embodiment of the lamp of theinvention. Herein, as in all Figures, similar parts have the samereference numerals.

The lamp comprises a lightguide with a first lightguide part 2-1 and asecond lightguide part 2-2. A prism 17, a circular polarizer and amirror 19 are also provided.

This lamp comprises a LED 1 that emits light into the first lightguidepart 2-1, without any extraction features. Hence, the light will beguided towards the opposite end. There, the light will enter the prism17, by which it will be totally reflected into the second lightguidepart 2-2, which does have extraction features 5. These will createvirtual images 10 of the LED, but now at a much larger perceiveddistance. The lamp, which is still very flat, now has a very largevirtual depth. Note that the prism 17 may also be integral with thefirst and second lightguide parts 2-1 and 2-2, to minimize losses.

Furthermore, circular polarizer 18 and mirror 19 co-operate to suppressreflections of ambient light. The circular polarizer 18 absorbs allincident light except for light with a certain (circular) polarization.Ambient light with this polarization is transmitted by the polarizer,and also by the lightguide parts 2-1 and 2-2, and finally is incident onmirror 19. This may flip the polarization of the light, which will now,after again passing the lightguide parts, be absorbed by the polarizer.Hence all ambient light is effectively absorbed. Since only half of thelight that is extracted by the structures 5 is absorbed by thepolarizer, the ratio between ambient (reflected) light and emitted lightis improved many times.

The feature of a side-lit lightguide with extraction features and acombination of a circular polarizer in front of the lightguide and amirror at the opposite side in general provides a lamp with suchsuppressed ambient reflections and an improved ratio of reflected andemitted light.

FIG. 5 shows another embodiment of the lamp of the invention, comprisingfirst and second (arrays of) LEDs 1-1 and 1-2, lightguides 2-1 and 2-2,extraction structures 5-1 and 5-2, creating virtual LED images 10-1 and10-2.

The principle of this lamp will be clear from the foregoing. Note thatthe lightguides have, at their respective end nearest the LEDs, a partthat is free from extraction structures, in order to prevent overlappingvirtual images.

The lamp shown here has an even larger (average) perceived depth,because at those locations where a single-lightguide lamp would show asmall depth, the second, overlapping lightguide takes over the creationof the virtual images. The perceived shape will be V-shaped, with theapex nearest the viewer in the center of the lamp.

On the basis of the principles as disclosed and illustrated above, theskilled person will be able to design many more variations.

FIG. 6 diagrammatically shows a cross-sectional view of anotherembodiment of a lamp according to the invention. This lamp has a highratio between reflected ambient light, which is effectively suppressed,and emitted light.

In this embodiment, the lamp comprises, apart from the by now well-knownparts, extraction structures 21 in the form of e.g. prismatic recessesfilled with birefringent material, and layers 22 of another birefringentmaterial.

The extraction structures 21 comprise a birefringent material, withfirst and second refractive indices for the two polarization axes. Oneof those refractive indices matches the refractive index of the materialof the lightguide 2, while the other is either lower or higher. In thiscase, the lightguide 2 could be made from glass, with n=±1.5, while thebirefringent material could be calcite, with respective refractiveindices of about 1.49 and 1.66. This means that rays with one type ofpolarization see only a perfect lightguide 2, and pass the extractionstructure without seeing it. In the Figure this is ray 23. The rays withthe other type of polarization, in the Figure ray 24, will be refracted,and thereafter extracted from the lightguide. However, the non-extractedlight will always, at any time, change its polarization, e.g. throughscattering at inevitable impurities or other surfaces. That light maythen be extracted. In this way, the lamp will emit all of the light witha single polarization only.

The incident ambient light is suppressed in the same way as describedabove, and the lamp comprises a circular polarizer and a mirror for thatreason. In order to be able to emit the linearly polarized light, suchas ray 24, the lamp comprises further birefringent material 22, eitherdeposited per structure 21, or (?) in the form of a patterned foil, etcetera. The material 22 rotates the polarization such that the light (?)can pass the polarizer 18. The retardation (or correspondingly thethickness) and polarization axis of the birefringent materials 21 and 22should of course be adapted to the polarization axis of the polarizer18.

The above embodiments are exemplary only, and should not be construed aslimiting the invention. They serve only as a means for betterunderstanding the invention.

1. A lamp for emitting light, comprising a transparent substantiallyplanar lightguide, having at least one light receiving side, a lightemission front surface and a back surface opposite the front surface,the back surface comprising a plurality of optical extractionstructures, and a plurality of light sources, positioned in an array andoptically coupled to at least one light receiving side, wherein the lampdoes not include any light-scattering structures in a path of the lightwhen emitted thereby, wherein the plurality of light sources comprise alinear light source, and a light division structure adjacent the linearlight source, arranged to divide a light emitting surface of the linearlight source into a plurality of light emitting sub-surfaces.
 2. Thelamp according to claim 1, wherein the array of light sources comprisesan arrangement, extending in at least two dimensions.
 3. The lampaccording to claim 2, wherein the extraction structures compriserecesses in the lightguide that are filled with a birefringent material,the lamp further comprising a layer of birefringent material disposedbetween the circular polarizer and the front surface.
 4. The lampaccording to claim 1, wherein the front surface is directly visible. 5.The lamp according to claim 1, wherein the lightguide comprises parallelfirst and second lightguide parts.
 6. The lamp according to claim 5,further comprising two arrays of light sources, wherein respective lightreceiving sides of the first and second lightguide parts are opticallyconnected to one of the two arrays of light sources.
 7. The lampaccording to claim 6, wherein the two arrays of light sources arearranged on opposite sides of the lamp.
 8. The lamp according to claim5, wherein the first lightguide part has a light receiving sidepositioned adjacent the array of light sources, and the secondlightguide part is optically connected to the first lightguide part bymeans of a mirror device.
 9. The lamp according to claim 5, wherein thefirst lightguide part is substantially free from extraction structures.10. The lamp according to claim 5, wherein the first and secondlightguide parts comprise a part at respective, mutually opposite lightreceiving sides thereof, that is free from extraction structures. 11.The lamp according to claim 1, further comprising two arrays of lightsources, and two lightguides with extraction structures and withopposite light receiving sides.
 12. A lamp for emitting light,comprising a transparent substantially planar lightguide, having atleast one light receiving side, a light emission front surface and aback surface opposite the front surface, the back surface comprising aplurality of optical extraction structures, and a plurality of lightsources, positioned in an array and optically coupled to at least onelight receiving side, wherein the lamp does not include anylight-scattering structures in a path of the light when emitted thereby,wherein at least one light source is moveable in at least one directionrelative to the lightguide.
 13. A lamp for emitting light, comprising atransparent substantially planar lightguide, having at least one lightreceiving side, a light emission front surface and a back surfaceopposite the front surface, the back surface comprising a plurality ofoptical extraction structures, and a plurality of light sources,positioned in an array and optically coupled to at least one lightreceiving side, wherein the lamp does not include any light-scatteringstructures in a path of the light when emitted thereby. The lampcomprises a circular polarizer positioned in front of the front surfaceand a mirror positioned adjacent the back surface.
 14. A lamp foremitting light, comprising a transparent substantially planarlightguide, having at least one light receiving side, a light emissionfront surface and a back surface opposite the front surface, the backsurface comprising a plurality of optical extraction structures, and aplurality of light sources, positioned in an array and optically coupledto at least one light receiving side, wherein the lamp does not includeany light-scattering structures in a path of the light when emittedthereby, wherein the lightguide has a retardation of less than 20 nm forlight traveling in the direction perpendicular to the exit surface. 15.A lamp for emitting light, comprising a transparent substantially planarlightguide, having at least one light receiving side, a light emissionfront surface and a back surface opposite the front surface, the frontsurface defining a plurality of optical extraction structures comprisingrecesses formed in the lightguide filled with a birefringent material,and at least one light source, positioned adjacent a light receivingside, and a substantially circular polarizer positioned in front of thefront surface, a mirror positioned adjacent the back surface, and alayer of birefringent material disposed between the polarizer and thefront surface.
 16. The lamp according to claim 15, wherein the lamp issubstantially free from light scattering structures in a path of thelight to be emitted.